1. Field of the Invention
This invention relates to an optical system, and particularly to an optical system having a reflecting optical element for forming the image of an object on the surface of a predetermined plane in a video camera, a still video camera, a copier or the like.
2. Related Background Art
Heretofore, optical systems comprised of refracting lenses alone are known as zoom optical systems, and these are generally such that refracting lenses provided with rotation-symmetrical spherical surfaces or rotation-symmetrical aspherical surfaces with respect to an optical axis are successively disposed in the direction of the optical axis and magnification change is effected by changing the spacing therebetween.
On the other hand, there is also known the zooming technique of moving a plurality of reflecting surfaces relative to one another to thereby vary the imaging magnification or focal length of a photo-taking optical system.
As shown in FIG. 6 of the accompanying drawings, in a cassegrainian reflector disclosed, for example, in U.S. Pat. No. 4,812,030, the spacing from a concave mirror 101 to a convex mirror 102 and the spacing from the convex mirror 102 to an imaging plane 103 are changed relative to each other to thereby effect magnification change.
FIG. 7 of the accompanying drawings shows another example disclosed in U.S. Pat. No. 4,812,030. An object beam 138 from an object impinges on a first concave mirror 131 and is reflected by the surface thereof and becomes a convergent beam and travels toward the object side and impinges on a first convex mirror 132, and is reflected toward the imaging plane side thereby and becomes a substantially parallel beam and impinges on a second convex mirror 134, and is reflected by the surface thereof and becomes a divergent beam and impinges on a second concave mirror 135, and is reflected thereby and becomes a convergent beam and is imaged on an imaging plane 137. The spacing between the first concave mirror 131 and the first convex mirror 132 is changed and also the spacing between the second convex mirror 134 and the second concave mirror 135 is changed to thereby change the focal length of an entire mirror optical system.
Also, in U.S. Pat. No. 4,993,818, the image formed by the Cassegrainian reflector shown in FIG. 6 is secondary-imaged by another mirror optical system provided at a rear stage, and the imaging magnification of this mirror optical system for secondary imaging is changed to thereby effect the magnification change of the entire optical system.
These optical system of the reflection type have required a great number of constituent parts, and to obtain necessary optical performance, it has been necessary to accurately assemble the respective optical parts. Particularly, the accuracy of the relative positions of the reflecting mirrors is severe and therefore, the adjustment of the position and angle of each reflecting mirror has been necessary.
As a method of solving this problem, there has been proposed, for example, a method of making mirror systems into a block to thereby avoid the incorporation errors of the optical parts occurring during the assembly thereof.
FIG. 8 of the accompanying drawings shows an embodiment of the reflecting optical system disclosed in Japanese Patent Application Laid-Open No. 09-258106 (corresponding to U.S. Pat. No. 5,999,311). A beam from an object passes through lenses (R1-R2) which are a first optical element B1 and a stop R3, and thereafter enters a second optical element B2. In the second optical element B2, the beam is refracted by a fourth surface R4, is reflected by a fifth surface R5, a sixth surface R6, a seventh surface R7 and an eighth surface R8, is refracted by a ninth surface R9, and emerges from the second optical element B2. At this time, the beam is primary-imaged on an intermediate imaging plane near the sixth surface.
Next, the beam enters a third optical element B3. In the third optical element B3, the beam is refracted by a tenth surface R10, is reflected by an eleventh surface R11, a twelfth surface R12, a thirteenth surface R13 and a fourteenth surface R14, is refracted by a fifteenth surface R15, and emerges from the third optical element B3.
Next, the beam enters a fourth optical element B4. In the fourth optical element B4, the beam is refracted by a sixteenth surface R16, is reflected by a seventeenth surface R17, an eighteenth surface R18, a nineteenth surface R19, a twentieth surface R20 and a twenty-first surface R21, is refracted by a twenty-second surface R22, and emerges from the fourth optical element B4. The beam having emerged from the fourth optical element B4 is finally imaged on an imaging plane R28, i.e., the photographing surface of an image pickup medium such as a CCD.
The movement of each optical element resulting from the magnification changing operation will now be described. In case of magnification change, the first optical element B1 which is a first optical unit, the stop R3, the third optical element B3 which is a third optical unit and a block B5 are fixed and are not moved. The second optical element B2 which is a second optical unit is moved in Z plus direction from the wide angle end toward the telephoto end in parallelism to the incidence reference axis of this optical element. Also, the fourth optical element B4 which is a fourth optical unit is moved in Z plus direction from the wide angle end toward the telephoto end in parallelism to the incidence reference axis of this optical element. A filter, cover glass and the twenty-eighth surface R28 which is the final imaging plane are not moved in case of focal length change.
The spacing between the second optical element B2 and the third optical element B3 is narrowed by the magnification change from the wide angle end toward the telephoto end, the spacing between the third optical element B3 and the fourth optical element B4 is widened, and the spacing between the fourth optical element B4 and the twenty-third surface R23 is widened.
That is, use is made of a plurality of optical elements comprising reflecting surfaces which are a plurality of curved surfaces and flat surfaces formed integrally with one another, and the relative position of at least two of the plurality of optical elements is appropriately changed to effect zooming, whereby the downsizing of the entire mirror optical system is achieved and yet the disposition accuracy (assembly accuracy) of the reflecting mirrors incidental to the mirror optical system is loosened.
Also, by adopting a construction in which the stop is disposed most adjacent to the object side of the optical system and the object image is formed at least once in the optical system, a reduction in the effective diameter of the optical system is achieved in spite of being a reflection type zoom optical system of a wide angle of view, and appropriate refractive power is given to a plurality of reflecting surfaces constituting the optical elements and the reflecting surfaces constituting each optical element are eccentrically disposed, whereby the optical path in the optical system is bent into a desired shape to thereby achieve the shortening of the full length of the optical system in a predetermined direction.
However, in the prior-art optical system comprising refracting optical elements alone, it is often the case that the entrance pupil is deep at the back of the optical system, and this leads to the problem that the greater is the spacing to the incidence surface located most adjacent to the object side as viewed from the stop, the larger becomes the effective diameter of the beam on the incidence surface with the enlargement of the angle of view.
Also, in both of the optical systems having the magnification changing function disclosed in the above mentioned U.S. Pat. No. 4,812,030 and the above-mentioned U.S. Pat. No. 4,993,818, the number of constituent parts such as reflecting mirrors and an imaging lens is great, and to obtain necessary optical performance, it is necessary to accurately assemble the respective optical parts. Particularly, the accuracy of the relative positions of the reflecting mirrors becomes severe and therefore, it is necessary to effect the adjustment of the position and angle of each reflecting mirror.
Also, in the optical system proposed in the above-mentioned Japanese Patent Application Laid-Open No. 09-258106, the downsizing of the entire mirror optical system is achieved and yet the disposition accuracy, i.e., assembly accuracy, of the reflecting mirrors incidental to the mirror optical system is loosened, but there is only a single lens on the object side of the stop and it is immovable and therefore, to make the F number constant during zooming, the diameter of the stop must be varied. If the F number is determined, the diameter of the stop is determined and therefore, when the size of image is small as in a still video camera, the diameter of a small stop necessarily becomes small. There also arises the problem that due to the cosine fourth power rule, the quantity of marginal light is greatly reduced. Also, all of the four elements constituting the optical system have negative optical power (optical power is the same meaning of the reciprocal number of the focal length), and this is not preferable in the correction of aberrations. Further, almost all of the surfaces constituting the third optical element which is a magnification changing portion have relatively strong positive power alone, and this is not preferable in the correction of aberrations.
The present invention has as its object to make, in an optical system having a reflecting optical element, the power arrangement of respective optical units appropriate to thereby make optical performance good.
In order to achieve the above object, an optical system according to an embodiment of the present invention is provided, in succession from the object side, with a first optical element of negative optical power, a second optical element of positive optical power and a third optical element of negative optical power, and is characterized in that at least one of the first to third optical elements is a reflecting optical element having a reflecting curved surface.
Also, an optical system according to another embodiment of the present invention is provided, in succession from the object side, with a first optical unit of negative optical power, a second optical unit of positive optical power and a third optical unit of negative optical power, and is characterized in that the positions of at least two of the first to third optical units relative to the imaging plane are changed to thereby effect zooming, and at least one of the first to third optical units has a reflecting optical element having a reflecting curved surface.